Instructions
Locate the hldsupdatetool.exe file on the computer that is hosting the server. If you do not remember where you placed the file, type its name in the search box at the bottom of the Start menu.
Right-click on the file and select "Properties" from the contextual menu.
Copy-and-paste or type the following parameters in the "Target" field:
Replace "X" with the code associated with the desired Valve game content and click "OK." For example, to add "Team Fortress 2" content, type "tf" in the target field. You can find a list of all game codes on the "General Server Setup" page at GarrysMod.com.
Double-click on the hldsupdatetool.exe file and wait for the program to finish downloading and installing the additional content. The window will automatically close once the process has been completed.
hldsupdatetool -game "X" -dir -command update